1. Field of the Invention
The present invention relates to a heat exchanger including a pair of headers and a plurality of parallel heat transfer tubes interconnecting the headers, and, more specifically, to a heat exchanger which is suitable for use in a vehicle air conditioner and which may achieve uniform distribution of a heat exchange medium.
2. Description of Related Art
In recent vehicle air conditioner configurations, particular condensers and evaporators have been employed to attain a heat exchanger which experience low pressure loss, and are capable of increasing the efficiency of heat exchange, but which facilitate manufacture of the air conditioner. In the field of condensers, so-called multi-flow type condensers, interconnecting a pair of header pipes with a plurality flat tubes, have been mainly employed. In the field of evaporators, stacking-type evaporators, consisting of a straight or U-shaped refrigerant path between a pair of header tanks, wherein such path is created by stacking a plurality of tubes formed by joining pairs of molded plates, have been mainly employed.
In a heat exchanger having headers, such as the above-described multi-flow type condenser or stacking-type evaporator, the pressure applied to each tube is first determined by the pressure gradient of refrigerant in an entrance side header, and the amount of refrigerant flowing into each tube is then determined by the degree of the refrigerant pressure in the header. Namely, in the header, the pressure near the refrigerant inlet portion of the header is highest, and the pressure gradually decreases as the distance from the inlet portion increases. Therefore, a large amount of refrigerant flows in the tubes near the refrigerant inlet portion, and the amount of refrigerant distributed to the tubes far from the refrigerant inlet portion is likely to be inadequate. Consequently, an area of inadequate refrigerant flow may be generated over the entire core portion of each of the above-described heat exchangers, and, as a result, the temperature distribution across the heat exchanger may become nonuniform and the efficiency of heat exchange may decrease.
In the case of a condenser, the condenser is positioned in front of an engine compartment of a vehicle, and the heat exchange is performed by introducing air for the heat exchange from a front grill of the vehicle. However, the opening area of the grill generally is not designed to be sufficiently large as compared with the area of the core portion of the condenser, to introduce air for heat exchange over the entire area of the core portion. Moreover, the introduction of air for heat exchange is further restricted by a bumper and a number plate. Under such conditions, a sufficient amount of air for heat exchange may be distributed only to a part of the entire core portion. Consequently, the entire core portion may not function for heat exchange at a high efficiency, and the efficiency of the heat exchanger may be reduced.
In the case of an evaporator, because generally a connecting portion is formed between a blower unit and an evaporator unit and both units are connected thereon; as in the case of a condenser, a sufficient amount of air for heat exchange may be distributed only to a part of the entire core portion of the evaporator. Consequently, the entire core portion may not function for heat exchange at a high efficiency, and the efficiency of the heat exchanger may be reduced.
In such conventional heat exchangers, in order to compensate for the reduced heat exchange performance due to deficiencies in the heat exchangers themselves and due to the problems caused by their location on a vehicle, partitions are provided in the headers, and thereby, refrigerant flow is divided in multiple paths in a heat exchanger, such as three paths or four paths, so that the refrigerant may comes into repeated contact with air passing through the heat exchanger.
Further, except the above-described multiple path structure formed by partitions, various structures for increasing the heat exchange performance, particularly, for improving the division of refrigerant flow in a heat exchanger, have been proposed.
For example, JP-A-58-140597 proposes to incline an inner fin in a heat transfer tube and lower the temperature difference between refrigerant in air entrance side and refrigerant in air exit side of a heat exchanger, thereby improving the heat transfer performance.
JP-A-9-196595 describes the insertion of a refrigerant introducing pipe into a header at a great depth, the pipe including refrigerant passing holes in the pipe for dividing a part of the flow of the refrigerant in the header. Consequently, the flow dividing condition is more uniform in the heat exchanger, and the cooling temperature is more uniform.
In the improvement due to the above-described multiple path structure, however, because at least two or three partitions are required, the cost for the material and the manufacture may increase, and the insertion hole processing for inserting the partitions into a header pipe or a header tank may be difficult.
Moreover, very difficult working and complicated designing are required to set the positions of the insertion holes, because the respective numbers of refrigerant tubes in the respective tube groups are divided by the partitions and the ratio of tube groups to partitions must be determined to be optimum, so that the efficiency for heat exchange may increase and refrigerant may flow more uniformly.
In the improvement of the above-described JP-A-58-140597 or JP-A-9-196595, although both propose to make the flow division in the heat exchanger more uniform, JP-A-58-140597 proposes accomplishing this only with the improvement of heat transfer tubes, and JP-A-9-196595 proposes accomplishing this only with the improvement of header portions.
Accordingly, the improvements of the above-described references have been examined by conducting tests only on tubes (corresponding to the heat transfer tubes described above) and only on headers, using those having shapes similar to the shapes proposed in the above-described references. As a result, although a slight improvement could be observed, a satisfactory result was not obtained.
Namely, as aforementioned, the amount of refrigerant flowing into each tube is determined by the pressure gradient of refrigerant in a header, in other words, by the degree of the refrigerant pressure in the header. Because the pressure near the refrigerant inlet portion of the header is highest and the pressure gradually decreases with the distance from the inlet portion, refrigerant flows in large amounts in the tubes near the refrigerant inlet portion, and the amount of refrigerant distributed to the tubes far from the refrigerant inlet portion is likely to be inadequate. Consequently, the flow division deteriorates, and the efficiency of heat exchange decreases. Satisfactory flow division and high efficiency for heat exchange are not achieved, so long as the essential problem of nonuniform flow division and decreased efficiency of heat exchange originating from the pressure distribution in the header, is not solved.